Spermidine can partially substitute for glucocorticoid in vivo, in relation to mammary cell differentiation. Mammary cells from mature virgin mice cannot synthesize milk-proteins under the influence of lactogenic hormones in the presence of inhibitors of DNA synthesis. The block in the formation of the milk-proteins is at a post-transcriptional level. Externalized mammary nuclei can synthesize DNA for about 90 minutes. Addition of dialyzed sera after this time can reactivate the DNA synthesis.